Relay Station, Mobile Station, Wireless Communication System, And Load Distribution Method

ABSTRACT

A relay station includes a load measuring unit configured to measure a load of the relay station; a load calculating unit configured to calculate, based on the load of the relay station and based on a load of an upstream base station or an upstream relay station directly connected to the relay station, a load reflecting a load of an upstream station; and a reporting unit configured to report the calculated load as load information to a mobile station or a downstream relay station.

FIELD

The embodiments discussed herein are related to a relay station, amobile station, a wireless communication system, and a load distributionmethod.

BACKGROUND

In a wireless communication system, a mobile station connects to thebase station affording the best radio quality among base stations in aservice area. Since connections to base stations change as mobilestations move around, the number of mobile stations connected to eachbase station does not remain constant but fluctuates. Consequently,imbalances in load occur among the base stations such as the number ofconnected mobile stations, the amount of data to be transmitted, andradio channel resource utilization. A base station with a heavy load hasproblems such as rejection of a connection request from a new mobilestation, deterioration of transmission rate, and timeout before data istransmitted completely.

In light of this, one load distribution method involves the exchange ofload information among base stations and instructing of a mobile stationconnected to a base station having a heavy load to perform handover(change of connection destination), so that the imbalance in load amongbase stations is resolved. Another method of load distribution involvesthe collection of load information from nearby base stations by mobilestations, where a mobile station that is connected to a base stationhaving a heavy load requests handover to another base station.

Such imbalances in load also occur in ad-hoc systems in which mobilestations form an autonomously distributed system. One load distributionmethod in this case involves the exchange of load information amongmobile stations and the selection of a mobile station to be connectedbased on loads of each mobile station (see, for example, JapaneseLaid-Open Patent Publication No. 2005-303828 (the Solution in theAbstract)).

Standards such as IEEE802.16j are investigating a system in which one ormore relay stations are inserted between a base station and a mobilestation. In this system, a mobile station is connected to a base stationdirectly or through one or more relay stations. Here, in a path betweena base station and a mobile station, the base station is locatedupstream from the mobile station, and the mobile station is locateddownstream from the base station. A relay station located towards thebase station is called an upstream relay station, and a relay stationlocated towards the mobile station is called a downstream relay station.

However, even though load distribution can be performed when a basestation reports a load thereof, further study is needed when a relaystation is taken into consideration. Even when a relay station reports aload thereof, efficient load distribution cannot be expected when heavyloads are present between the relay station and an upstream base stationor between the relay station and an upstream relay station.

SUMMARY

According to an aspect of an embodiment, a relay station includes a loadmeasuring unit configured to measure a load of the relay station; a loadcalculating unit configured to calculate, based on the load of the relaystation and based on a load of an upstream base station or an upstreamrelay station directly connected to the relay station, a load reflectinga load of an upstream station; and a reporting unit configured to reportthe calculated load as load information to a mobile station or adownstream relay station.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a wireless communication systemaccording to a first embodiment.

FIG. 2 is a diagram illustrating a relay station according to the firstembodiment.

FIG. 3 is a flowchart of a process for reporting load informationaccording to the first embodiment.

FIG. 4 is a diagram explaining an example of reporting the loadinformation according to the first embodiment.

FIG. 5 is a diagram explaining another example of reporting the loadinformation according to the first embodiment.

FIG. 6 is a flowchart of a process of load calculation according to thefirst embodiment.

FIG. 7 is a flowchart of a load distribution process according to thefirst embodiment.

FIG. 8 is a diagram illustrating a relay station according to a secondembodiment.

FIG. 9 is a diagram illustrating one example of a typical radio channelframe.

FIG. 10 is a diagram illustrating one example of a typical messageformat.

FIG. 11 is a diagram illustrating a conventional radio channel frame.

FIG. 12 is a diagram illustrating a conventional message format for arelay station.

FIG. 13 is a diagram illustrating a radio channel frame of a relaystation according to the second embodiment.

FIG. 14 is a diagram illustrating a message format of the relay stationaccording to the second embodiment.

FIG. 15 is a diagram illustrating another example of a radio channelframe for the relay station according to the second embodiment.

FIG. 16 is a diagram illustrating another example of a message formatfor the relay station according to the second embodiment.

FIG. 17 is a diagram illustrating a mobile station according to thesecond embodiment.

FIG. 18 is a diagram illustrating a load distribution process accordingto the second embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained withreference to the accompanying drawings. In the explanation below, likereference numerals are used for like items, and repetitive explanationsare omitted. Embodiments described here do not limit the invention.

FIG. 1 is a diagram illustrating a wireless communication systemaccording to a first embodiment. As depicted in FIG. 1, the wirelesscommunication system according to the first embodiment includes two ormore base stations (BS), one or more relay stations (RS), and one ormore mobile stations (MS). As an example, FIG. 1 depicts five basestations BS#1 through BS#5, four relay stations RS#1 through RS#4, andsix mobile stations MS#1 through MS#6. For RS#1, BS#1 is an upstreamstation. For BS#1, RS#1 is a downstream relay station. For RS#3, RS#4 isa downstream relay station. For RS#4, RS#3 is an upstream relay station.

Depending on the location of a mobile station, the mobile station isconnected directly to a base station (the relationship between MS#1 andBS#1), or is connected to a base station through a relay station (therelationship among MS#2, RS#1, and BS#1). A relay station is directlyconnected to a base station (the relationship between RS#1 and BS#1), oris connected to a base station through another relay station (therelationship among RS#4, RS#3, and BS#5). Accordingly, a mobile stationmay be connected to a base station through multiple relay stations (therelationship between MS#5 and BS#5).

A relay station may be located in an area enabling wireless connectionto multiple base stations or other relay stations. Corresponding to themovement of a mobile station, the mobile station may be located in anarea enabling wireless connections to multiple base stations or multiplerelay stations. FIG. 1 illustrates a situation where one station can beconnected to multiple stations. In FIG. 1, solid lines between stationsindicate actual connections between stations. Broken lines betweenstations indicate that stations are not actually interconnected but canbe interconnected. Central frequencies or frequency widths of radiochannels may be identical or different.

Not particularly limited hereto, but in the explanation below, thewireless communication system is assumed to be an IEEE802.16 orthogonalfrequency division multiple access (OFDMA) system. In the IEEE802.16OFDMA system, multiple relay stations or mobile stations share a radiochannel resource made up of a time axis and a frequency axis. Notparticularly limited hereto, but in the explanation below, a timedivision duplex (TDD) scheme and a D&F (Decode & Forward) scheme areassumed as a duplex communication scheme and a relay scheme,respectively.

FIG. 2 is a diagram illustrating a relay station according to the firstembodiment. As depicted in FIG. 2, a relay station 11 according to thefirst embodiment includes antennae 12 and 13, a first wirelesstransceiver unit 14 for communicating with upstream stations, a secondwireless transceiver unit 15 for communicating with downstream stations,a load collecting unit 16, a load measuring unit 17, a control unit 18,a storage unit 19, and a load calculating unit 20.

The load collecting unit 16 collects, through the antenna 12 and thefirst wireless transceiver unit 14, load information reported from aconnected upstream station and nearby upstream stations. The loadmeasuring unit 17 measures a load of the relay station 11. The load ofthe relay station 11 may be the number of mobile stations directlyconnected to the relay station 11 or the number of other downstreamrelay stations, the amount of data to be transmitted, or radio channelresource utilization, or any combination of two or more.

The load calculating unit 20 calculates loads to be reported todownstream stations based on the load known from the load informationcollected by the load collecting unit 16 and based on the load measuredby the load measuring unit 17. The first wireless transceiver unit 14receives, through the antenna 12, signals from the connected upstreamstation and the nearby upstream stations, demodulates and decodes thesignals, and measures radio quality with respect to the upstreamstations, based on the signals. The radio quality may be the receivedpower or the signal-to-interference-plus-noise power. The first wirelesstransceiver unit 14 also encodes and modulates signals to be transmittedto the connected upstream station and transmits the signals through theantenna 12.

The second wireless transceiver unit 15 reports the load calculated bythe load calculating unit 20 as load information of the relay station11. The second wireless transceiver unit 15 receives signals fromdownstream stations connected to the relay station 11 and performs areceiving process such as demodulation and decoding. The second wirelesstransceiver unit 15 also encodes or modulates signals to be transmittedto downstream stations connected to the relay station 11 and transmitsthe signals through the antenna 13.

The control unit 18 selects an upstream station based on the radioquality measured by the first wireless transceiver unit 14 and the loadinformation collected by the load collecting unit 16. The control unit18 manages, for downstream stations, the connections and allocates radiochannels. The control unit 18 reports information concerning the manageddownstream stations to upstream stations or requests radio channelallocation from the upstream stations. The storage unit 19 storesinformation concerning the managed downstream stations.

Instead of the first and the second wireless transceiver units 14 and 15performing processes for upstream stations and downstream stations inparallel, one wireless transceiver unit may perform processes forupstream and downstream stations by time-sharing. Multiple wirelesstransceiver units may perform processes for upstream stations, multiplewireless transceiver units may perform processes for downstreamstations, etc.

FIG. 3 is a flowchart of a process for reporting load informationaccording to the first embodiment. As depicted in FIG. 3, when the relaystation 11 starts the process for reporting load information, the loadcollecting unit 16 collects load information reported by upstreamstations (step S11). The load measuring unit 17 measures a load of therelay station 11 (step S12). The load calculating unit 20 calculatesloads to be reported to downstream stations based on the loads of theupstream stations acquired at step S11 and the load of the relay station11 measured at step S12 (step S13). An example of load calculation willbe described later. The loads calculated at step S13 are reported as theload information of the relay station 11 through the second wirelesstransceiver unit 15 and the antenna 13 (step S14), and the reportingprocess ends. The reporting process is periodically conducted.

FIG. 4 and FIG. 5 are diagrams explaining examples of reporting the loadinformation according to the first embodiment. As depicted in FIG. 4,for example, the load information is reported as a signal point 32 of anI/Q constellation 31. Beforehand, each signal point 32 of the I/Qconstellation 31 is correlated with a value of load. The relay station11 selects a signal point that is the nearest to the load calculated bythe load calculating unit 20 and reports the point. A base station mayreport load information in a similar way. Another way of reporting maybe as depicted in FIG. 5, the load information may be reported as amessage. In this case, the load information uses, for example, aninformation element 37 in a message 36 and is reported as a numericalvalue. The load information of a base station may be reported in asimilar way.

FIG. 6 is a flowchart of a process of load calculation according to thefirst embodiment. As depicted in FIG. 6, when the calculation of loadbegins at step S13 (see FIG. 3), the load calculating unit 20 applies anoffset value to the loads of the upstream stations acquired by the loadcollecting unit 16 or the loads of the relay station 11 acquired by theload measuring unit 17 and provides a weighted value (step S21). Thisweighting process is performed as needed. For example, when a downstreamstation selects an upstream station taking account of factors other thanload, the weighting process is performed.

The offset value may be constant or variable. For instance, as an offsetvalue, a variable based on the radio quality of a signal received froman upstream station or based on frequency of handover may be used. As anoffset value, a constant value based on a delay due to a route via arelay station may be used.

The load calculating unit 20 compares the loads of the upstream stationsacquired by the load collecting unit 16 with the loads of the relaystation 11 acquired by the load measuring unit 17 (step S22). If theweighting process is performed at step S21, values weighted with theoffset value are compared. If the loads of the upstream stations arelarger (step S22: YES), the loads of the upstream stations are selectedas the load information of the relay station 11 (step S23) and thecalculating process is completed. If the load of the relay station 11 islarger (step S22: NO), the load of the relay station 11 is selected asthe load information of the relay station 11 (step S24), and thecalculating process is completed. In this example, the selection oflarger loads reflects the loads of the upstream stations. Loads may beadded to acquire the sum of loads as the load information in order toreflect the loads of the upstream stations.

FIG. 7 is a flowchart of a load distribution process according to thefirst embodiment. The load distribution process is performed at therelay stations and the mobile stations depicted to be downstreamstations in the wireless communication system of FIG. 1. The structureand the operation of a relay station have been explained with referenceto FIG. 2 through FIG. 6. The structure of a mobile station is similarto a conventional device that possesses a function for distributingloads.

As depicted in FIG. 7, when the load distribution process begins at adownstream station, the downstream station receives signals coming froma connected upstream station and nearby upstream stations and measuresthe radio quality of each signal (step S31). The downstream stationcompares each radio quality measured at step S31 with the radio qualityof a signal received from a currently connected upstream station todetermine whether handover can be conducted (step S32). Whether handoveris to be performed may be determined based on the comparison of eachradio quality measured at step S31 with a given threshold.

If there is no upstream station that can accept the handover (step S32:NO), the process returns to step S31. If there is an upstream stationthat can accept the handover (step S32: YES), the downstream stationcollects load information that the upstream station, i.e., the target ofthe handover, reports (step S33). From the load information, thedownstream station can know the load of the upstream station. The loadof the upstream station reflects loads of stations further upstream.

The downstream station compares the load of the upstream station(handover target) with the load of the currently connected upstreamstation, and determines whether to conduct load distribution (step S34).If it is determined that handover to the upstream station (handovertarget) can be expected to improve transmission rate (step S34: YES),handover from the currently connected upstream station to the upstreamstation (handover target) is conducted (step S35), and the loaddistribution process is ended. If handover to the upstream station(handover target) cannot be expected to improve the transmission rate(step S34: NO), the process returns to step S31.

According to the first embodiment, the relay station 11 reports loadinformation that reflects loads of other stations including a basestation to which the relay station 11 is directly connected, or anotherrelay station directly connected to the relay station 11 and the basestation. In this way, the loads of upstream stations can be relayed todownstream stations. Therefore, in the wireless communication system,because a downstream station can perform load distribution reflectingthe loads of upstream stations, the load distribution is more efficient.

For instance, in the wireless communication system depicted in FIG. 1,loads of base stations or relay stations are assumed to change from 0(minimum) to 100 (maximum) and loads of the base station BS#1, the relaystation RS#1, and the base station BS#2 are assumed to be 100, 25, and75, respectively. In this example, the relay station RS#1 compares 25with 100 of the base station BS#1, the connected upstream station. Therelay station RS#1 selects 100, the larger load, and reports the load asload information for the relay station RS#1. When the mobile stationMS#3 is within an area enabling connection with either the relay stationRS#1 or the base station BS#2, the mobile station MS#3 compares 100, theload reported from the relay station RS#1, with 75, the load reportedfrom the base station BS#2. The mobile station MS#3 selects, as anupstream station for connection, the base station BS#2 having thesmaller load value.

A second embodiment relates to a wireless communication system includingstructure information that defines a frame structure within a radiochannel frame. In this wireless communication system of the secondembodiment, an upstream station reports, using the structureinformation, load information reflecting loads of stations furtherupstream. A downstream station estimates, from the structureinformation, loads of the upstream stations (at least wireless devicesthat wirelessly communicate with a mobile station) and performs loaddistribution.

FIG. 8 is a diagram illustrating a relay station according to the secondembodiment. As depicted in FIG. 8, a relay station 41 according to thesecond embodiment differs from the relay station 11 according to thefirst embodiment in that a load estimating unit 42 replaces the loadcollecting unit 16, and a structure information adjusting unit 43 isfurther included. The load estimating unit 42 receives, through theantenna 12 and the first wireless transceiver unit 14, structureinformation reported from a connected upstream station and nearbyupstream stations, and based on the structure information, estimates theloads of the upstream stations. The load calculating unit 20 calculatesa load to be reported to a downstream station, based on the loadsestimated by the load estimating unit 42 and the loads measured by theload measuring unit 17.

The structure information adjusting unit 43 reflects the loadscalculated by load calculating unit 20 on the structure information ofthe relay station 41. The control unit 18 selects an upstream stationfor connection, based on the radio quality measured by the firstwireless transceiver unit 14 and the loads estimated by the loadestimating unit 42. Other components are similar to the firstembodiment. With reference to FIG. 9 through FIG. 16, a method forreporting load information using the structure information will beexplained.

FIG. 9 is a diagram illustrating one example of a typical radio channelframe. As depicted in FIG. 9, a radio channel frame 51 is made up of adownstream channel sub-frame 52 and an upstream channel sub-frame 53.Downstream channel means a radio channel from an upstream station to adownstream station, and upstream channel means a radio channel from adownstream channel to an upstream channel.

The downstream channel sub-frame 52 includes areas of a preamble (P), amap (MAP), and a burst (B). The preamble (P) is a signal with a knownpattern used for a base station, a relay station, and a mobile stationto be synchronized. The map (MAP) is a message for defining a framestructure such as a burst point in the radio channel frame 51, burstlength, a modulation type of burst, and a coding type, and for reportingthe frame structure to a downstream relay station or mobile station. Themessage corresponds to the structure information. The burst (B) is ablock for transmission data sent to a downstream relay station or mobilestation.

In the example depicted in FIG. 9, in the downstream channel, the basestation (BS#1) or the relay station (RS#1) transmits the preamble (P),the map (MAP), and the burst (B) to the relay station (RS#1) or themobile station (MS#1, MS#2). The burst (B) for the relay station (RS#1)includes a burst for the relay station (RS#1) and, a burst for themobile station (MS#2) connected to the relay station (RS#1) or a burstfor a downstream relay station. A base station or relay station maytransmit a burst, designating in the map (MAP) for a mobile station orrelay station further downstream and connected to a downstream relaystation.

The relay station (RS#1) transmits, among bursts (B) received from theupstream base station (BS#1) or an upstream relay station, a burst (B)for a downstream relay station or the mobile station (MS#2) at the nextor a subsequent frame. Consequently, a map (MAP) that the relay station(RS#1) transmits can be different from a map (MAP) that the upstreamstation (BS#1) connected to the relay station (RS#1) transmits.

A downstream relay station or the mobile station (MS#2) directlyconnected to the relay station (RS#1) synchronizes the relay station(RS#1) using the preamble (P). Usually, when a relay station or mobilestation is present in an area enabling the relay station or mobilestation to establish communication with multiple base stations or relaystations, the relay station or mobile station distinguishes the basestations or relay stations based on the known pattern of the preamble(P) and selects a base station or relay station affording the best radioquality.

FIG. 10 is a diagram illustrating one example of a typical messageformat. As depicted in FIG. 10, a message 61 of the map (MAP) includes amessage type 62, the number of relay stations and mobile stations (totalnumber of stations) 63 that transmit or receive data within a framedefined by the map (MAP), an identifier (station identifier) 64 for therelay stations or mobile stations, a burst position 65 and a burstlength 66 for the relay stations or mobile stations. As a result, themap (MAP) becomes longer along with an increase in the number of relaystations or mobile stations that transmit or receive data within oneframe. In FIGS. 10, 12, 14, and 16, designation such as a modulationtype of each burst or an encoding type is omitted.

A relay station analyzes all the contents of the map (MAP) of anupstream base station or relay station. Consequently, the relay stationcan estimate a load of an upstream station with the load estimating unit42. For instance, the relay station estimates a load of an upstreamstation based on factors such as the total number of stations or the sumof burst lengths within the map (MAP) of each frame, or based on thenumber of different station identifiers within a given number of frames.These factors may be replaced with an average over a given period. Thesum of burst lengths can be acquired as radio channel resourceutilization, a proportion of a burst to the radio channel resource(frame length). The radio channel resource (frame length) can beacquired from a frame length and a frequency range. The total number ofstations or the number of different station identifiers within a givennumber of frames is identical to the number of relay stations and mobilestations directly connected.

FIG. 11 and FIG. 12 are diagrams each illustrating a conventional radiochannel frame and a conventional message format for a relay station. Asdepicted in these figures, a relay station, from among bursts (B)transmitted from a connected upstream station in a downstream channelsub-frame 71, transmits only a burst (B) for a relay station directlyconnected to the relay station or a mobile station (MS#2) and does nottransmit part of bursts indicated by a broken line in FIG. 11. As aresult, the total number of stations or the sum of burst lengthsincluded in the message 72 of the map (MAP) becomes smaller incomparison with the connected upstream station. Namely, the relaystation has not reported a heavy load of the connected upstream stationto a downstream station. In light of this, in the second embodiment, arelay station reports a load of the connected upstream station to thedownstream station as described below.

FIG. 13 and FIG. 14 are diagrams illustrating one example of a radiochannel frame and a message format of a relay station according to thesecond embodiment. As depicted in these figures, a relay station, fromamong bursts (B) transmitted from a connected upstream station in adownstream channel sub-frame 74, transmits bursts for a downstreamstation and a mobile station (MS#2), but a message 75 of a map (MAP)includes a hypothetical relay station or mobile station as a dummy. Thestructure information adjusting unit 43 of the relay station 41 controlsthe message 75 of the map (MAP) so that such a dummy is included. Aburst (B) for the dummy is indicated by a broken line in FIG. 13 and isnot transmitted. The hypothetical relay station or mobile station isconsidered as, for example, a relay station or mobile station that isnot present in the area but can express loads of upstream stations.

FIG. 15 and FIG. 16 are diagrams illustrating another example of a radiochannel frame and a message format for a relay station according to thesecond embodiment. As depicted in these figures, a relay station, fromamong bursts (B) transmitted from a connected upstream station in adownstream channel sub-frame 77, transmits a burst (B) for a downstreamrelay station and a mobile station (MS#2) and does not transmit burstsin a part indicated by a broken line in FIG. 15. However, a message 78of the map (MAP) is identical to a message of the map (MAP) transmittedfrom the connected upstream station. The structure information adjustingunit 43 of the relay station 41 controls the message 78 of the map (MAP)so that a message of the map (MAP) transmitted from the connectedupstream station is transferred as it is to a downstream station. Thismethod is applicable to a situation where a base station or relaystation expresses in the map (MAP), a relay station or mobile stationfurther downstream and connected to a downstream relay station.

Whether a dummy is included in a message of the map (MAP), or a messageof the map (MAP) of the connected upstream station is transferred as itis, the relay station does not transmit an unnecessary burst (B) thatmay interfere with other cells and can relay a heavy load of theconnected upstream station to a downstream station that is connected tothe relay station. The load estimating unit 42 of the relay station 41can estimate a load of an upstream station, based on the total number ofstations or the total burst length included in the map (MAP).

FIG. 17 is a diagram illustrating a mobile station according to thesecond embodiment. As depicted in FIG. 17, a mobile station 81 includesan antenna 82, a wireless transceiver unit 83, a load estimating unit84, a control unit 85, a microphone unit 86, a speaker unit 87, a keyedinput unit 88, a display unit 89, and a storage unit 90. The loadestimating unit 84 receives, through the antenna 32 and the wirelesstransceiver unit 83, structure information reported from a connectedupstream station or nearby upstream stations and estimates loads of theupstream stations, based on the structure information. The wirelesstransceiver unit 83 receives, through the antenna 82, signalstransmitted from the connected upstream station or nearby upstreamstations and measures radio quality for an upstream station, based onthe received signals. The control unit 85 selects an upstream stationfor connection, based on the radio quality measured by the wirelesstransceiver unit 83 and the loads estimated by the load estimating unit84.

The microphone unit 86, the speaker unit 87, the keyed input unit 88,the display unit 89, and the storage unit 90 are commonly known and thusthe detailed explanation is omitted. The mobile station 81 is, forexample, a mobile phone. The mobile station 81 may be configured bycombining a device such as a wireless connection card including thewireless transceiver unit 83, the control unit 85, and the storage unit90 with a personal computer functioning as the keyed input unit 88, thedisplay unit 89, and the storage unit.

The mobile station 81 analyzes the contents of the map (MAP) of anupstream base station or relay station in a similar manner as the relaystation 41 according to the second embodiment. The load estimating unit84, based on the total number of stations in the map (MAP) or the numberof relay stations and mobile stations directly connected, or based onthe total burst length in the map (MAP), can estimate a load such as theradio channel resource utilization with respect to an upstream station.

FIG. 18 is a diagram illustrating a load distribution process accordingto the second embodiment. The load distribution process is performed bya relay station and mobile station that function as downstream stationsin the wireless communication system depicted in FIG. 1. Regarding thestructure and operation of the relay station, see FIG. 8 through FIG. 16and the explanation thereof. Regarding the structure and operation ofthe mobile station, see FIG. 17 and the explanation thereof.

As depicted in FIG. 18, the load distribution process of the seconddiffers from the first embodiment in that at step S43, the downstreamstation receives the structure information reported in each frame fromthe upstream station (handover target) and based on the structureinformation, estimates a load of the upstream station (handover target).Regarding the measuring of the radio quality (step S41), determiningwhether to conduct handover (step S42), determining whether to conductload distribution (step S44), and conducting handover (step S45), seestep S31, step S32, step S34, and step S35 of the first embodiment.

According to the second embodiment, without adding a new signal ormessage for reporting load information, a load of an upstream stationcan be relayed to a downstream station. As a result, in a wirelesscommunication system, the downstream station can perform loaddistribution reflecting the loads of upstream stations, therebyrealizing more efficient load distribution.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A relay station comprising: a load measuring unit configured tomeasure a load of the relay station; a load calculating unit configuredto calculate, based on the load of the relay station and based on a loadof an upstream base station or an upstream relay station directlyconnected to the relay station, a load reflecting a load of an upstreamstation; and a reporting unit configured to report the calculated loadas load information to a mobile station or a downstream relay station.2. The relay station according to claim 1, further comprising: a loadestimating unit configured to estimate a load of a base station oranother relay station based on structure information that defines astructure of a radio channel frame; and a structure informationadjusting unit configured to reflect the load calculated by the loadcalculating unit on the structure information, wherein the loadcalculating unit acquires the load of the upstream base station or theupstream relay station directly connected to the relay station from theload estimated by the load estimating unit.
 3. The relay stationaccording to claim 2, wherein the structure information adjusting unitadjusts the structure information so that a mobile station or adownstream relay station directly connected to the relay station, and ahypothetical relay station or a hypothetical mobile station are regardedas stations connected to the relay station.
 4. The relay stationaccording to claim 2, wherein the structure information adjusting unitadjusts the structure information to include structure informationreported from the upstream base station or the upstream relay stationdirectly connected to the relay station.
 5. A mobile station comprisinga load estimating unit configured to estimate a load of an upstreamstation based on structure information that defines a structure of aradio channel frame and reflects a load of an upstream base station oran upstream relay station directly connected to the mobile station. 6.The mobile station according to claim 5, further comprising a unitconfigured to change a base station or a relay station connecteddirectly to the mobile station, based on the load estimated by the loadestimating unit.